Use of valves to control intake to and exhaust from a combustion chamber in an internal combustion engine has long been known. It is common for the valves to be actuated by a cam device which times the opening and closing of the valves with respect to the cycle of the engine. These cams are typically driven by the power shaft of the engine through a drive train which times the cam relative to the rotational position of the power shaft and thereby with respect to the engine cycle. The location of the cam with respect to the power shaft and the valves can dictate the design of the valves and the cam drive train,
Thus, in the design of an internal combustion engine, the location of the cam relative to the valve arrangement and the power shaft is an important design consideration. The relative complexity, the compactness, the number of components, and the packaging into the engine of both the valve arrangement and the cam drive train are all affected by the relative location of the cam. In turn, these design parameters for the valve arrangement and the cam drive train directly affect the compactness and reliability, which are often high priorities in internal combustion engines. Additionally, portability is often a high priority for small engines and, again, will be directly affected by the above-mentioned design parameters.
One common cam arrangement is a straddle-mounted cam shaft having a drive gear or sprocket rotationally fixed to one end. The drive gear or sprocket is driven by the power shaft through a gear train, timing chain, or timing belt. While such cam shafts are capable of satisfactory performance, they do not necessarily lend themselves to compact and/or high reliability engine design. For example, the length of the cam shaft and the straddle-mounting can make the packaging of the cam shaft within the engine problematic. Often, the packaging of such cam shafts dictates that the cam shaft be located remote from the power shaft. Such remote location can increase the complexity of the cam shaft drive train, thereby decreasing its reliability. If a gear train is used, several gears may be required to span the distance between the power shaft and the remotely located cam shaft. Reliability can be affected by the build-up of inaccuracies caused by the backlash at each mesh point in the gear train. Further, each gear represents a potential single point failure in the drive train. Similarly, if a timing chain is used, a relatively long chain may be used to span the distance between the power shaft and the remotely located can shaft. Again, each link represents an additional inaccuracy, as well as an additional single point failure in the drive train.
A unique cam arrangement has been used in radial aircraft engines to satisfy the special requirements presented by the radial orientation of the engine cylinders. Such engines have employed a ring cam driven by the engine power shaft through a planetary gear train consisting of a sun gear mounted on the power shaft, a plurality of planet gears mating with the sun gear, and a ring gear mating with the planet gears. This arrangement is employed in radial aircraft engines because it is desirable to mount the cam coaxially with the crankshaft so that the cam is equidistant from the valve arrangements associated with each of the cylinder heads.
While such cam arrangements are capable of satisfactory performance in radial aircraft engines, they do not necessarily lend themselves to compact and reliable engine design. For example, the use of the planetary gear train requires that the ring cam be mounted coaxial with the power shaft. This coaxial configuration can put severe limits on the sizing of the cam and the associated planetary gear train, as well as the location, type, and orientation of the valve arrangement. Further, the use of planet gears can reduce the reliability of the engine due to the error introduced at the mesh points of the planet gears and the potential single-point failure represented by the planetary gear set. Additionally, the use of planet gears adds additional weight and complexity to the engine and takes up space within the engine which potentially could be utilized for other engine components, thereby directly affecting the compactness and portability of the engine.
Thus, it can be seen that there is a need for a compact cam design which can be incorporated within an internal combustion engine minimizing the number of dedicated components and allowing for high reliability of the engine.